1. Field of the Invention
The present invention is directed to a shock wave generator for use in an extracorporeal lithotripsy unit.
2. Description of the Prior Art
Shock wave generators are known in the art, for example as described in German OS 33 12 104, having a flat coil connectable to a high voltage supply and disposed opposite a membrane which closes a housing filled with a shock wave transmissive fluid, such as water. The membrane has a carrier plate consisting of an electrically insulating material, and has an electrically conductive section disposed on one side of the carrier. The membrane is connected to the housing at its edge, with the electrically conductive section being insulated from the windings of the flat coil.
The high voltage supply for shock wave generators of this type includes a capacitor chargeable to several kilovolts, for example 20 kV. The energy stored in the capacitor is rapidly discharged into the coil, so that the coil generates a magnetic field extremely rapidly. At the same time, a current is induced in the electrically conductive section of the membrane. This current being opposite to the current flowing in the coil, and consequently generating an opposing magnetic field. The interaction of the two magnetic fields causes the electrically conductive section of the membrane and the carrier plate connected thereto to be rapidly repelled from the coil. This movement generates a shock wave in the fluid-filling housing, which is focused in a known manner to the calculi, for example, kidney stones, in the body of a life form, and effects disintegration thereof.
The membrane in conventional shock wave generators of this type is secured to the housing by rigidly clamping the edge of the carrier. When the membrane is driven to generate a shock wave, the membrane is thus exposed to sudden bending stresses which can result in over-stressing of the membrane, and ultimately in failure thereof. To alleviate these effects, the electrically conductive section of the membrane in conventional shock wave generators is annular. This results in a reduced mechanical stressing of the electrically conductive section of the membrane, however, the carrier of the membrane is still exposed to considerable stresses, so that the risk of rupture is particularly high at the edge of the carrier, because of the rigid clamping at that location.
To achieve an optimal conversion of the electrical energy generated by the high voltage supply into impact energy, it is necessary in conventional shock wave generators to attach the electrically conductive section of the membrane as close as possible to the flat coil. Such placement is subject to limitations, however, because a certain insulating distance must be maintained to avoid voltage arcing between the membrane and the coil. For this purpose, an insulating foil is sometimes interposed between the membrane and the coil. Voltage arcing deteriorates the effectiveness of the shock wave generator, and additionally can lead to damage of the membrane, thereby diminishing the useful life of the unit.
It is standard in conventional shock wave generators to connect the electrically conductive section of the membrane to ground potential together with one terminal of the coil. This standard practice requires such a large distance to be present between the electrically conductive section of the membrane and the flat coil in such conventional shock wave generators that an unsatisfactory efficiency in the conversion of electrical energy into impact energy results.
In such conventional shock wave generators, a cavitation problem also exists as a consequence of the relatively high speed of the membrane within the fluid which occurs during the generation of shock waves. Such cavitation can cause pitting of the membrane, thus contributing to its premature failure.